Figure 172 from Chapter 5 (Endoplasmic Reticulum) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. Electron micrograph of a Nissl body in a large motor neuron. The fenestrated cisternae of the endoplasmic reticulum are separated by intervals of 0.2 to 0.5 µm which are crowded with free polyribosomal aggregates. Image by Sanford Palay. A PDF copy of the accompanying chapter is available on the ASCB’s BioEDUCATE website.